1. Field of the Invention
This invention is generally related to microscopic analysis of structures and components of solid materials and more specifically to a method and apparatus for directly imaging and distinguishing among atomic species on the surface of a solid material.
2. Description of the Prior Art
It has been a long-time goal of scientists and researchers to be able to "see" or directly image individual atoms of a substance, particularly of a solid material, with enough resolution to not only spatially resolve their respective locations in relation to each other, but also to distinguish and identify exact atomic species and bonding characteristics on a real-time and a real-space basis. Prior to this invention, no such method or apparatus was capable of doing so.
The most primitive technique of analyzing materials used simple visual observation and the sense of touch. Optical microscopes provided much greater optical resolution for smaller particles and features; and various hardness comparisons and other physical, chemical, spectral, and electrical property analyses were developed to improve recognition of material characteristics. However, even with the best optical instruments, the limits of direct, optically enhanced, visual resolution were reached many years ago.
More sophisticated devices, such as electron microscopes, were developed to achieve better resolution of tiny particles or features, but vertical resolution is limited, and direct atomic imaging remained beyond the resolution limits of electron microscopes. Other devices, such as secondary ion mass spectroscopy (SIMS) and low-energy electron diffraction (LEED), have been developed with capabilities of resolving surface structures and distinguishing between atomic and molecular species present in the solid material. However, such materials detection and analysis devices do not provide real-space visual images of the individual atomic species in the material.
The introduction of scanning tunneling microscopy (STM) in the early 1980s has provided both the spatial resolution and the analytic capabilities to image atoms on surfaces of solid materials. U.S. Pat. No. 4,343,993, issued to G. Binning et al. in 1982, describes such STM apparatus, which is now available commercially from at least four manufacturers, including Microscience, Inc., of Norwell, Massachusetts; McAllister Technical Services of Berkeley, California; RHK Instruments of Rochester Hills, Michigan; and V. G. Instruments, of Danvers, Massachusetts. The article entitled "Analysis and Characterization of Thin Films: A Tutorial" by Lawrence L. Kazmerski, published in Vol. 24, pages 387-418 of Solar Cells, based on a paper presented at the 8th Photovoltaic Advanced Research and Development Project Review Meeting in Denver, Colorado, November 15-18, 1987, shows how such STM apparatus can be used to show the effects of hydrogen processing in borondoped silicon grain boundaries and to provide three-dimensional molecular imaging of a CuInSe.sub.2 crystal surface. An even more recent article entitled "STM Studies of Molecular and Chemical Properties of Surfaces" by Mark J. Cardillo, published in the January 1989 issue of physics. Today shows similar three-dimensional STM images of a silicon crystal surface before and after reaction with NH.sub.3, which are produced in simulations based on molecular orbital calculations and demonstrates a way that STM can provide a picture of the electron density of specific orbitals of surface molecules.
However, the above-described STM apparatus and analysis techniques still lack the ability to differentiate directly between individual atomic species present in an image or to indicate their bonding to other atoms. Therefore, those STM apparatus and techniques still have not fulfilled the goal of being able to directly image the individual, respective atomic species and their bonding that exist in the surface layer of a solid material on a real-time analysis basis.